It is estimated that nearly three million people in the United States have glaucoma and more than one hundred thousand people are blind from glaucoma. Glaucoma is the second leading cause of blindness in adult Americans age eighteen to sixty-five and the leading cause of blindness in African Americans.
Glaucoma is an optic neuropathy (a disorder of the optic nerve) that usually occurs in the setting of an elevated intraocular pressure. An increase in intraocular pressure may result in changes in the appearance (“cupping”) and function (“blind spots” in the visual field) of the optic nerve. If the pressure remains high enough for a long enough period of time, total vision loss may occur.
The eye is a hollow structure that contains a clear fluid called “aqueous humor.” Aqueous humor is continuously produced in the posterior chamber of the eye by the ciliary body. The fluid passes around the lens, through the pupillary opening in the iris and into the anterior chamber of the eye. Once in the anterior chamber, the fluid drains out of the eye through two different routes. In the “uveoscleral” route, the fluid percolates between muscle fibers of the ciliary body. The primary pathway for aqueous outflow in humans is through the “canalicular” route that involves the trabecular meshwork and Schlemm's canal. The trabecular meshwork and Schlemm's canal are located at the junction between the iris and the cornea called “the angle.” The trabecular meshwork is composed of collagen beams arranged in a three-dimensional sieve-like structure and lined with a monolayer of trabecular cells. The outer wall of the trabecular meshwork coincides with the inner wall of Schlemm's canal, which is a tube-like structure that runs around the circumference of the cornea. The aqueous fluid travels through the spaces between the trabecular beams into the Schlemm's canal, through a series of collecting channels that drain from Schlemm's canal and into the episcleral venous system. In a normal situation, aqueous production is equal to aqueous outflow and intraocular pressure remains fairly constant in the 10 to 21 mmHg range. High pressure develops in an eye because of an internal fluid imbalance. In glaucoma, the resistance through the canalicular outflow system is higher than normal causing reduced outflow thereby causing an internal fluid imbalance and resulting in an increased pressure.
In primary open angle glaucoma, the drainage angle formed by the cornea and the iris remains open, but the microscopic drainage channels in the trabecular meshwork are at least partially obstructed. Other forms of glaucoma (such as angle closure glaucoma and secondary glaucomas) involve decreased outflow through the canalicular pathway due to mechanical blockage, inflammatory debris, cellular blockage, etc.
When the drainage system does not function properly, the aqueous humor cannot filter out of the eye at its normal rate. As the fluid builds up, the intraocular pressure within the eye increases. The increased intraocular pressure compresses the axons in the optic nerve, which carries vision from the eye to the brain, and also may compromise the vascular supply to the optic nerve. Damage to the optic nerve is painless and slow and a vision loss can occur before a person is even aware of a problem.
Eye and systemic medications are used to treat open angle glaucoma by decreasing the production of aqueous humor or increasing its drainage from the eye. Surgical treatment may be performed when medication fails to lower the intraocular pressure. For example, surgical procedures are used to either open up the anatomically closed irido-corneal angles or create a new drainage pathway of the aqueous humor outside the eye.
Trabeculectomy is a surgical procedure that creates a pathway for aqueous fluid to escape to the surface of the eye. The anterior chamber is entered beneath the scleral flap and a section of deep sclera and trabecular meshwork is excised. Post-operatively, the aqueous fluid passes through the resulting hole and collects in an elevated space beneath the conjunctiva. The fluid then is either absorbed through blood vessels in the conjunctiva or traverses across the conjunctiva into the tear film. A deficiency of such procedure is that it creates a pathway for bacteria that normally live on the surface of the eye and eyelids to get into the eye.
Another surgical procedure involves the use of an aqueous shunt. A full thickness hole is made into the eye at the limbus, usually with a needle. The shunt is inserted into the eye through this hole and aqueous humor drains out to the surface of the eye. Many complications are associated with aqueous shunts. A thickened wall of scar tissue may resist outflow and limit the reduction in eye pressure. The resulting unrestricted flow through the shunt to the outer surface may result in too low of an intraocular pressure and can damage the eye in different ways that could lead to loss of function and sight. As such shunts are open to the surface of the eye, a pathway is created for bacteria to get into the eye and endophthalmitis can occur.
Laser surgery is a surgical procedure to reduce the intraocular pressure and includes cyclophotocoagulation (reducing the production of aqueous humor by using a laser to freeze the part of the eye that produces aqueous humor), iridotomy (use of a laser to make a hole in the iris to allow fluid to flow more freely in the eye), and trabeculoplasty (use of a laser to create holes in the drainage area of the eye to allow fluid to drain more freely). However, laser surgery is complex and suffers from a variety of deficiencies.